Piezoelectric resonators are primarily used in RF filters and oscillators. RF filters are increasingly being used in mobile communications devices. These resonators are commonly referred to as bulk acoustic wave (BAW) resonators. Other acronyms for the same or similar devices include FBAR (thin-film bulk acoustic resonators), SMR (solidly mounted resonators), TFR (thin film resonators), or SCR (stacked crystal resonators). The resonators are interconnected, typically at the upper metal level, to build RF filters.
It is known that a bulk acoustic wave (BAW) resonator in general comprises a piezoelectric layer sandwiched between two electronically conductive layers that serve as electrodes. When a radio frequency (RF) signal is applied across the device, it produces a mechanical wave in the piezoelectric layer. BAW resonators are typically fabricated on silicon (Si), gallium arsenide (GaAs), glass, or ceramic substrates. BAW resonators are typically manufactured using various thin film manufacturing techniques, such as for example sputtering, vacuum evaporation or chemical vapor deposition. BAW resonators utilize a piezoelectric thin film layer for generating the acoustic bulk waves. The resonance frequencies of typical BAW resonators range from 0.5 GHz to 5 GHz, depending on the size and materials of the device. BAW resonators exhibit the typical series and parallel resonances of crystal resonators. The resonance frequencies are determined mainly by the material of the resonator and the dimensions of the layers of the resonator.
A typical BAW resonator consists of an acoustically active piezoelectric layer, electrodes on opposite sides of the piezoelectric layer, and an acoustical isolation from the substrate. Although the resonant frequency of a BAW device also depends on other factors, the thickness of the piezoelectric layer is a predominant factor in determining the resonant frequency. As the thickness of the piezoelectric layer is reduced, the resonance frequency is increased.
The material used to form the electrode layers is an electrically conductive material. The acoustical isolation is produced with a substrate via-hole, a micromechanical bridge structure, or with an acoustic mirror structure. In the via-hole and bridge structures, the acoustically reflecting surfaces are the air interfaces below and above the devices. The bridge structure is typically manufactured using a sacrificial layer, which is etched away to produce a free-standing structure. Use of a sacrificial layer makes it possible to use a wide variety of substrate materials, since the substrate does not need much modification, unlike with the via-hole structure. A bridge structure can also be produced using an etch pit structure, in which case a pit is etched in the substrate or the material layer below the BAW resonator to produce the free standing bridge structure.
FIG. 1 illustrates a cross-section of a conventional piezoelectric resonator. The piezoelectric resonator includes a substrate 10, an acoustic mirror or acoustic reflector 20, a bottom electrode 30, a piezoelectric layer 40, and a top electrode 50. The top electrode can be constructed from several metallic and dielectric layers. The bottom electrode 30 can also be constructed from several metallic and dielectric layers; molybdenum is typically used. The top electrode 50 shown in FIG. 1 includes a bottom metal layer 52 of a material with high acoustic impedance, and a top metal layer 54 of a material with low impedance.
FIGS. 2-5 illustrate conventional fabrication steps to form the top electrode 50. As illustrated in FIG. 2, the bottom metal layer 52 is deposited on the piezoelectric layer 40. The top metal layer 54 is then deposited upon the bottom layer 52. The top metal layer 54 is then patterned, selectively to under-laying layer 52 (FIG. 3) and resist removed. Bottom metal layer 52 is then etched using top metal layer 54 as a hard mask (FIG. 4). As a result, a portion 56 of the bottom metal layer 52 is also etched forming a recess underneath the top metal layer 54 (FIG. 5). Additionally, when an interconnect metal layer is fabricated to make contact with the top electrode 50, further undercutting of the top layer 54 can occur during an etch step of the interconnect metal layer. In either case, undercutting of the top metal layer 54 negatively impacts the performance of the piezoelectric resonator.
During fabrication of conventional piezoelectric resonators, such as the process illustrated in FIGS. 2-5, problems arise during etching of the bi-layer top electrode and during etching of the metallization layer that contacts the top electrode bi-layer stack. The difficulty is finding etches that do not preferentially attack the bottom metal layer 52 causing undercut voiding at the device periphery. This is particularly true if wet etching is employed because of various galvanic and catalytic reactions that do occur.